Varicella or chickenpox is a generalized illness that occurs due to primary infection with varicella zoster virus (VZV). It is usually a self limiting and benign illness in children. However, chickenpox is not without risks, even for children of this age; severe, complicated, and occasionally fatal varicella occur in previously healthy children, as well as the immunocompromised. Following varicella, latent infection with VZV develops in dorsal root ganglia and cranial nerve ganglia. For the lifetime of most individuals the virus remains quiescent, but about 30% go on to manifest clinical zoster, usually in the 6th to the 8th decade of life when cell mediated immunity (CMI) to VZV wanes due to normal aging. (1)There is little data on the health burden of varicella in developing countries. A seroprevalence study from India reported 15% seronegativity and susceptibility to varicella in adults (2).

Varicella vaccine :
Live attenuated varicella vaccine was developed in Japan in the early 1970s as a means to prevent varicella primary infection with varicella-zoster virus (VZV) in healthy and immunocompromised individuals. It has been shown to be greater than 95% effective in protecting against severe disease and 70% to 90% effective against mild to moderate illness for children 1 to 2 years of age for at least 7 to 9 years after vaccination. Clinical trials indicated that the Takahashi vaccine strain Oka was highly immunogenic and safe. It seems likely that varicella vaccine is not only effective in preventing varicella but also in preventing zoster. (1) Children and adults who experience breakthrough infection with wild varicella-zoster virus usually demonstrate mild disease, with an average of fewer than 50 lesions. (3)

Dosing :
A routine two-dose schedule of varicella vaccination of children is now recommended along with a second-dose catch-up varicella vaccination for children, adolescents, and adults who previously had received only one dose. The basis for these changes was: the recognition that vaccine failures occur after a first dose; outbreaks of varicella had been reported in populations with high coverage with one dose of vaccine. (3) A group of 138 children in New York, Tennessee, and California were tested for seroconversion after receiving 1 dose of the vaccine licensed in the United States, using the fluorescent antibody to membrane antigen (FAMA) assay, only 76% of these children seroconverted. (4) These results were one of the reasons that a second dose of varicella vaccine was mandated in 2006 by the Centers for Disease Control and Prevention (CDC) for all children. At present, studies in the US are underway to determine whether the second dose confers more protection than only 1 dose. (1)

Immunization schedule :
Children < 13 years of age can be given two doses of varicella-containing vaccine, with the first dose administered at 12 to 15 months of age and the second dose at 4 to 6 years of age (i.e., before a child enters kindergarten or first grade). The second dose can be administered at an earlier age provided the interval between the first and second dose is at least 3 months. (3)

Side effects :
Varicella vaccine induces mild varicelliform rash and fever in 5% to 10% of recipients. Vaccine virus rarely can be transmitted from healthy patients who experience rash. More serious adverse events (e.g., encephalitis, ataxia) have been reported rarely. (3)

Contraindications :
Varicella vaccine is contraindicated in people who have a blood dyscrasia, except for children with acute lymphocytic leukemia; in people with primary or acquired immunodeficiency (including immunodeficiency due to HIV infection), in pregnant women, and in people who have had an anaphylactic reaction to varicella vaccine or any component, including gelatin. However, vaccine may be given to people with humoral immunodeficiency and may be considered for people with asymptomatic or mildly symptomatic HIV infection with age-specific CD4+ T-lymphocyte percentages greater than 15% after the risks and benefits have been weighed. (3)

Varicella combination vaccine
MMRV vaccine is indicated for simultaneous vaccination against measles, mumps, rubella, and varicella among children 12 months through 12 years of age. (1)

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